This Decades-Old Rally Tech Was So Advanced McLaren Still Uses It Today

The world of rally went crazy in the early 2000s. Technical regulations were fairly open, and with no road-car homologation requirement and ever-better computer technology, the stage was set for truly wild machines. Peugeot was the first to really take advantage of the rules with the 206 WRC, but in 2003, corporate stablemate Citroën took things further with suspension technology so clever, McLaren still uses it today.

This story starts further back than the turn of the millennium, though. In the late Eighties, Australian Chris Heyring, then an art professor, came up with an idea for a new type of automotive suspension. His “Kinetic” suspension system seeks to overcome the limitations of traditional anti-roll bars. An anti-roll bar, obviously being a fixed piece of metal connecting opposite sides of the car, is of a fixed stiffness. Some traditional units have a range of adjustments, but once it’s bolted into place, roll stiffness is fixed. You're stuck with a set "moment (torque) resisting body roll per degree of body roll," as Race Car Vehicle Dynamics defines roll stiffness, also known as "roll rate."

Heyring’s solution hydraulically interlinks opposite sides of the car. Essentially, the dampers in a Kinetic-suspension car have separate top and bottom units. The compression units on one side of the car are linked via flow-restriction valves and hydraulic lines to the rebound units of the other side, and vice versa. This essentially creates a hydraulic anti-roll bar. When a wheel compresses, the piston in the damper pushes up on the compression unit, and the fluid in that top unit flows across the car to the rebound unit on the other side, pushing the wheel up towards the body. The stiffness of the roll bar is governed by an accumulator, a metal sphere with fluid and nitrogen separated by a flexible membrane. You can compress a gas, but not a fluid, so when there’s excess fluid in the system, it presses up against the membrane, compressing the nitrogen. (The accumulator operates with the same basic principle as a suspension sphere in a hydropneumatic Citroën, like a DS.) The valves between the damper units and the hydraulic lines regulate damping force.

(Engineering Explained’s, Jason Fenske has a nice visual explanation of the system as used by McLaren, if you’re looking to get a better image of how all this works)

Essentially, the Kinetic suspension decouples vertical wheel motion (compression and rebound) from horizontal motion (roll), while offering a ton of bandwidth when it comes to roll stiffness. Heyring developed the system for off-roaders, where to increase wheel articulation, you really want no anti-roll bar whatsoever. But on the road, going without an anti-roll bar is unpleasant at best, and dangerous at worst. Some Jeep Wranglers have an anti-roll bar disconnect for off-roading, but the bar must be reconnected for driving on the road.

Kinetic, the company Heyring founded to develop the system was acquired in 1999 by American supplier Tenneco. Citroën was up against the dominant Peugeot team, and it thought that the Kinetic suspension system, or at least a version of it, would give its Xsara WRC an edge over the dominant 206 WRC. The Xsara WRC incorporated a sort of split metal anti-roll bar supported by the hydraulic system. It offered Citroën’s rally drivers a car with great flexibility—on rough terrain, each wheel had the bandwidth to handle the many dips, crests, bumps, jumps, rocks, and everything else you see on a rally stage. On faster, smoother roads, you could effectively ramp up roll stiffness for more speed.

This also helped keep the aerodynamic platform more stable. It’s tricky to manage the airflow on a race car that’s constantly moving around, even on a perfectly smooth piece of track—it’s why modern F1 cars, especially in the new ground-effect era, must be so stiff. On the many varied stages of the World Rally Championship, a rally car moves around a hell of a lot more, unlike in a circuit race car, you need wheel travel too. How do you create a nice level aerodynamic platform while also keeping the car soft enough to deal with, say, the dusty stages of the Acropolis Rally? For Citroen, the Kinetic system was the answer, and it was a key part of the machine that turned the team into a WRC powerhouse.